28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

POSTER

Effects of Fenced Habitat Protection on Annual and Perennial Plants in the Western Mojave Desert

M. Brooks¹, K. Berry², J. Proctor¹, and L. Shapiro¹
U.S. Geological Survey, Western Ecological Research Center, ¹Las Vegas Field Station, 160 N. Stephanie St., Henderson NV 89014; ²Box Springs Field Station, 22835 Calle San Juan de Las Lagos, Moreno Valley, CA 92553

Two major forms of anthropogenic disturbance, livestock grazing (mostly by sheep) and off-highway vehicle (OHV) use, have been prevalent in the western Mojave Desert since the late 1800s and 1960s respectively. These disturbances have been cited as reducing plant cover, biomass, and species diversity, and promoting plant invasions, but the effects of subsequent removal of these disturbances on Mojave Desert plant communities are mostly unknown. In 1973, the 10,100 ha Desert Tortoise Research Natural Area (DTNA) was closed to OHVs, and in 1976 it was closed to livestock grazing. Effective habitat protection from these disturbances began in 1980 with the completion of a 1m tall fence around the perimeter.

Fenced habitat protection has two effects on plant communities: it allows recovery from past disturbances and provides protection from additional future disturbances that adjacent unprotected areas continue to be subject to. Habitat protection can also be coupled with active restoration, although this is not the case at the DTNA where disturbance exclusion is the only management treatment inside the fenceline. The DTNA provides an opportunity to test the hypothesis that fenced habitat protection from human disturbances can have positive effects on plant community structure in the western Mojave Desert.

Above-ground live biomass and species richness of annual plants were measured during April 1990, 1991, and 1992. More species of annual plants had higher biomass inside than outside the DTNA during each year. Biomass of goldfields (Lasthenia californica) and comb-bur (Pectocarya spp.) were significantly higher inside in 1990, small-flowered poppy (Eschscholzia minutiflora) and goldfields were higher inside in 1991, and (Amsinckia tessellata), Mojave suncup (Camissonia campestris) and lacy phacelia (Phacelia tanacetifolia) were higher inside in 1992. The only species with higher biomass outside was the alien Mediterranean grass (Schismus spp.) during each of the three years. Annual plants were also sampled during 1994 and 1995. Biomass of red brome (Bromus madritensis ssp. rubens), Mediterranean grass, and total alien annual biomass were significantly higher in unprotected areas during at least one of two years. In no cases were alien species significantly more abundant inside the DTNA.

Density and cover of perennial shrubs were measured in June 1990, and cover, height, volume, and diversity were measured in June 1995. Total density of perennial shrubs was generally unaffected by protection, whereas total cover and diversity (Shannon-Wiener index) were 33-50% higher in the protected area. Cover of burrobush (Ambrosia dumosa), California buckwheat (Eriogonum fasciculatum), boxthorn (Lycium andersonii), and Fremont dalea (Dalea fremontii) were all significantly higher in protected areas. Because cover was higher inside the DTNA, and density was generally unaffected by protection, the average size of individual shrubs was higher inside the DTNA. Height diversity, cover diversity, and volume diversity of perennial shrubs were not significantly affected by protection.

Seedbank biomass in the top 2cm of soil was measured in April 1990, 1991, and 1992. Biomass was higher inside than outside the DTNA during each year, but high intersample variance limited significant differences to 1992.

These data support the hypothesis that fenced habitat protection can have positive effects on plant community structure in the western Mojave Desert after only 10-15 years of protection. These benefits should be considered when evaluating the wide range of effects of habitat protection for threatened or endangered species such as the desert tortoise (Gopherus agassizii), the Mohave ground squirrel (Spermophilus mohavensis), or the many listed plants species that inhabit this region.